1. Field of the Invention
The present invention relates to an analog level shifter formed in a semiconductor integrated circuit. In particular, the present invention relates to an analog level shifter having a CMOS type operational amplifier.
2. Description of the Related Art
A known conventional analog level shifter is described in, for example, FIG. 7 of Y. Miyawaki et al., “A 29-mm2, 1.8-V-only, 16-Mb DINOR Flash Memory with Gate-Protected-Poly-Diode (GPPD) Charge Pump,” IEEE Journal of Solid-State Circuits, Vol. 34, No. 11, November 1999.
In the analog level shifter described in this document, an input voltage Vref is supplied to an operational amplifier. Then, a level-shifted voltage VO is obtained which is given by:VO=VN+Vref×(R2/R1)  (1)
If a CMOS operational amplifier is used as the operational amplifier, the input voltage Vref is normally supplied to a gate electrode of an NMOS transistor. Accordingly, the input voltage Vref must be higher than a threshold voltage of the NMOS transistor. If the input voltage Vref is lower than the threshold voltage of the NMOS transistor, the output voltage VO does not have the value shown by Equation (1).
With progress in semiconductor processing technologies, MOS transistors have been increasingly fine-grained. Further, the operating voltages of circuits and thus voltage levels to be handled have been reduced. However, owing to the need for a reduction in off leak current, the threshold voltage of the NMOS transistor can only gradually be reduced compared to a decrease in supplied voltage. As a result, it is difficult to convert the level of a low analog voltage.
In spite of a low input voltage Vref, a PMOS input type operational amplifier is sometimes used in order to meet the relationship shown in Equation (1). In the PMOS input type operational amplifier, the input voltage Vref is supplied to a gate electrode of the PMOS transistor. However, three amplification stages including a final one are required to allow such a PMOS input type operational amplifier to operate correctly. Thus, with the PMOS input type operational amplifier, it is difficult to ensure stable operations. Further, a pattern area and an operating current increase.